Prior Art
It is common practice in sewage treatment to aerate the raw or partially treated sewage or industrial effluent in an aeration tank to promote reduction of the organic matter by aerobic microorganisms present in the sewage. The mixed liquor from the aeration zone is then transferred to a clarifier where the particulate matter, carrying with it the microorganisms, settles to the bottom as sludge. In the activated sludge process, a portion of the sludge from the clarifier containing the microorganisms is reintroduced into the aerator along with the raw sewage to intensify the biological activity.
During aeration, a portion of the carbonaceous waste is oxidized to carbon dioxide which escapes into the atmosphere. The remaining portion is converted to biomass which remains as particulate matter in the mixed liquor that is discharged from the aeration zone. Nitrogen containing compounds, such as ammonia, are converted to nitrates by the biological activity induced by aeration. These nitrates are also discharged from the aeration zone in the mixed liquor. Until recently, the nitrates, along with the phosphates that are present in the waste, were merely discharged in the effluent from the clarifier. However, it has now been recognized that these nitrates and phosphates produce overfertilization in natural water resources, such as lakes and streams, into which the effluent containing these nutrients is discharged. Such overgrowth cannot only be unsightly but can be harmful to the ecological balance of the water and can be especially harmful to the fish population.
Present technology for removing nitrogen from sewage or other liquid streams includes oxidizing the nitrogen present in the waste to its most highly oxidized state, namely nitrate, and then reducing the nitrate to nitrogen gas. When biological oxidation and reduction is used it is referred to as the biological nitrification-denitrification process. Other potential processes are air stripping of ammonia gas and removal of ammonia or nitrate ions by ion exchange.
However, it is generally agreed that the biological nitrification-denitrification process is the most practical and economical. The usual approach is to oxidize the ammonia, nitrogen and organic nitrogen to nitrates in either a single stage or two stage system utilizing air or oxygen as the oxygen source. During the oxidization, carbon is removed by oxidation to carbon dioxide or conversion to carbonaceous solids prior to or in conjunction with the nitrogen oxidation. Following nitrification, it is then necessary to denitrify by removing the oxygen source in the presence of anaerobic bacteria. Since carbon is removed in the nitrification stage, it may be necessary to add supplemental carbon during the denitrification. Usually, methanol or a similar product is added to the denitrification stage as a supplemental carbon feed.
Much of the apparatus presently available for the biological denitrification of sewage requires large or extensive power-driven mechanisms of various kinds. According to the present invention, apparatus is provided which is more compact and wherein the extensive mechanism heretofore required can be dispensed with, and the need to add carbon by the introduction of a carbon compound, usually methanol, is eliminated or substantially reduced, particularly after the apparatus has been operated for a period of time.